The present invention relates to a surgical cutting instrument. More particularly, it relates to a surgical cutting instrument, such as a bone-cutting bur, capable of high speed operation and minimal interference with surgical site visibility.
Surgical cutting instruments, such as those incorporating a bur, are commonly used to perform a variety of procedures. For example, many neuro-otological surgical operations entail partial or total removal of bone or other hard tissue via a bur or other cutting tip rotating at high speeds. Exemplary procedures in this field include cochleostomies, removal of acoustic neuroma tumors, removal of the scutum in a tympanoplasty, etc. Numerous other surgical operations have similar bone/hard tissue cutting or removal requirements.
The typical surgical cutting instrument is akin to a drill, including a drill handpiece that rotates a cutting implement. The handpiece houses a motor and a chuck or other adaptor, with the chuck being rotated by the motor under the control of a foot- or finger-operated switch. The cutting implement normally includes a cutting tip (e.g., bur) attached to or formed by a cutter shaft that is otherwise connectable to the handpiece chuck. To provide a clearer view of the surgical site, the cutter shaft is normally elongated to position the cutting tip away from the handle. To this end, if the elongated shaft is unsupported by a separate external sleeve, bur “wobble” inevitably occur and safety concerns are raised by having a large length of exposed shaft rotating at high speeds. If the rotating shaft comes in contact with a nerve or other critical anatomy, serious injuries can occur. Thus, support sleeves are commonly employed.
More particularly, the cutter shaft is disposed within an elongated support sleeve that otherwise extends from a forward end of the housing. The cutter shaft is adapted to be inserted into the sleeve so that a proximal end of the shaft rotatably and releasably engages the chuck. The cutter shaft/support sleeve is commonly referred to as a “bur extender”. To provide for high speed concentric rotation of the cutting implement relative to the support sleeve, most surgical cutting instruments employ a ball bearing assembly between the outer support sleeve and the inner cutter shaft at a distal end thereof. While this design can readily operate at speeds in excess of 20,000 RPM, an outer diameter of the support sleeve must be relatively large (on the order of 6 mm) to accommodate the ball bearing assembly. This larger outer dimension, in turn, impairs surgical site visibility, and increases overall costs.
Conventional surgical cutting instrument designs raise additional line-of-sight and handling concerns. In order to support relatively high rotational speeds, most surgical cutting instruments employ a straight bur extender. Unfortunately, with this straight configuration, the support sleeve will often times be in or near the surgeon's line of sight upon desired positioning of the cutting tip at the surgical site, thus impeding the surgeon's view of the surgical site. On a related point, the relatively large outer diameter and/or straight bur extender may affect the surgeon's ability to position the cutting tip at a desired location, especially when the cutting instrument is used in conjunction with a microscope.
One known technique for addressing the line of sight problem described above is to extend the support sleeve/cutter shaft at an angle relative to a central axis of the handpiece. While this technique may improve visibility, handling of the device can be cumbersome as the angular extension initiates immediately adjacent the handpiece, with the bur extender itself remaining straight. With conventional designs, the angled configuration is usually accomplished via beveled gears rotating off-axis from each other. Thus, the angle formed by the bur extender relative to the handpiece axis must be a relatively large distance away from the cutting tip due to the need for the chuck mechanism to be on the same axis as the rotating cutting tip. As a result, only a slight lateral off-set between the cutting tip and the handpiece axis can be achieved, thus minimizing the effect on visibility issues.
In light of the above, it would be desirable to locate the angle or bend away from the handpiece, closer to the cutting tip, such as with a curved bur extender. To this end, one attempt at providing a surgical cutting instrument having a curved bur extender is described in U.S. Pat. No. 4,811,736. While highly viable, this design is potentially limited in the available rotational or cutting speed. In particular, the construction and material selection for the support sleeve and cutter shaft may limit the maximum, viable operational speed to less than 20,000 RPM. This potential limitation may be due in part to the bearing design utilized with the cutting instrument. In particular, U.S. Pat. No. 4,811,736 describes a plastic sleeve bearing 52 disposed within a distal end of an outer support sleeve 33. A cylindrical journal 42 component of the cutting implement (or “bur assembly”) is mounted within, and rotates relative to, this plastic sleeve bearing 52. Unfortunately, the additional plastic sleeve bearing 52 component may give rise to failures at high speeds due to excessive heat. Further, an overall diameter of the outer support sleeve 33 must be large enough to accommodate the separate sleeve bearing 52, thus negatively affecting visibility during use. Commercial applications of the teachings of U.S. Pat. No. 4,811,736, such as a drill instrument available from Medtronic-Xomed of Jacksonville, Fla., under the tradename “Skeeter,” are not highly stiff.
The above-described surgical cutting instruments often times require additional steps to complete many surgical procedures. For example, a mastoidectomy entails exposing the mastoid periosteum and then carefully drilling/removing the mastoid bone using a cutting/burring instrument and microscope. With a conventional bur extender that is angled relative to the handpiece, but is otherwise straight and employs a ball bearing assembly between a relatively large diameter outer support tube and the cutter shaft, it is highly difficult for the surgeon to visually see the bur cutting tip against the mastoid bone. As such, drilling of the mastoid bone entails first briefly contacting the rotating bur tip against the mastoid bone at an estimated optimal position, and then retracting the bur tip. Once retracted, the surgeon visually determines whether the bur tip was optimally positioned relative to the mastoid bone. If so, the bur tip is returned to the previous point of contact and drilling is commenced, with periodic stoppages to allow the surgeon to visually confirm that the procedure is proceeding as desired. If the initial contact point is less than optimal, the bur tip is repositioned relative to the mastoid bone, and the process repeated. Conversely, with a surgical cutting instrument akin to that described in U.S. Pat. No. 4,811,736, the inherent rotational speed limitations require use of several, differently sized burs. For example, a first, relatively large diameter bur (on the order of 6-7 mm) is initially used to de-bulk a portion of the mastoid bone. Subsequently, a second, smaller diameter bur (on the order of 4-5 mm) is used to remove an additional portion of the mastoid bone. Once visualization of the target site is overtly impaired by this second bur, a third, even smaller diameter bur (on the order of 2 mm) is employed to complete the procedure.
Surgical cutting instruments continue to be important tools for a multitude of surgical procedures. Unfortunately, prior art surgical cutting instruments are characterized as either high speed with poor visibility or lower speed with improved visibility. Therefore, a need exists for a surgical cutting instrument designed for long-term, high-speed operation with minimal impact on user visibility, minimized heat build-up, and improved stiffness.